Fungus resistant gypsum-based substrate

ABSTRACT

The present invention relates to a fungus resistant gypsum board, made of first and second polymeric fibrous sheets with a gypsum core sandwiched there between. The gypsum core containing less than 0.03% of formulation additives that serve as fungus nutrients and less than 0.5% of the dry gypsum core contains of fungus nutrients. The fibrous sheets are preferably nonwovens and the gypsum core preferably contains a fungicide. The invention is also directed to a process for making a fungus resistant gypsum board.

[0001] This application claims benefit of priority from ProvisionalApplication No. 60/121,697 filed on Feb. 25, 1999, and from ProvisionalApplication No. 60/121,698 filed on Feb. 25, 1999.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to gypsum-based construction materials.More particularly, the invention relates to a fungus resistantgypsum-based substrate faced with a synthetic polymeric sheet materialthat is suited for use as a construction material such as wallboard orceiling panels. The invention is also directed to a process formanufacturing a fungus resistant gypsum-based substrate faced with asynthetic polymeric sheet material.

[0004] 2. Description of Related Art

[0005] Fungi frequently can be found in the walls of buildings. Commonfungi include mold and mildew. Fungi are especially troublesome in wallswith poor ventilation where moisture can become trapped in the wall. Thewalls of portable buildings, such as temporary classrooms, have provedparticularly susceptible to fungus growth because water often seeps inaround the openings and joints of such structures. In buildings withpoor ventilation or inefficient heating and air conditioning systems,the building walls are more likely to become breeding grounds forfungus. Some funguses that grow in walls, such as the stachybotryschartarum (atra) fungus, produce toxins that have been known to renderstructures uninhabitable.

[0006] Conventional gypsum-based construction materials have thedisadvantage that they support fungus growth when used in moistenvironments. Fungus needs both moisture and nutrients to survive.Naturally occurring organic matter that is a part of conventional gypsumboard products, such as cellulose, paper fibers, starch, andcontaminants, serve as nourishment for many strains of fungus.Accordingly, when conventional gypsum board becomes chronically moist orwater damaged due to excessive humidity, water leaks, condensation, orflooding, fungus will grow on or in the gypsum board. Fungus growth canbe exacerbated in gypsum board walls when vinyl wall coverings are usedon the interior surface of the walls. Such vinyl wall coverings can trapmoisture inside the gypsum board where it facilitates fungus growth.

[0007] Gypsum wallboard and gypsum panels are traditionally manufacturedby a continuous process. In this process, a gypsum slurry is firstgenerated in a mechanical mixer by mixing calcium sulphate hemihydrate(also known as calcined gypsum), water, and other agents. The gypsumslurry is normally deposited on a paper sheet. The gypsum slurry mayinclude additives such as cellulose fibers that help to strengthen thegypsum core once it is dry. Starch is conventionally added to the gypsumslurry in order to improve the adhesion between the gypsum core and thepaper facing. An upper continuously advancing paper sheet is laid overthe gypsum and the edges of the upper and lower paper sheets are pastedto each other with a starch paste. The paper sheets and gypsum slurryare passed between parallel upper and lower forming plates or rolls inorder to generate an integrated and continuous flat strip of unsetgypsum sandwiched between the paper sheets that are known as facing orliners. This strip is conveyed over a series of continuous moving beltsand rollers for a period of 2 to 5 minutes during which time the corebegins to hydrate back to gypsum and hardens. During each transferbetween belts and/or rolls, the strip is stressed in a way that cancause the paper facing to delaminate from the gypsum core if theadhesion between the gypsum core and the facing is not sufficient. Oncethe gypsum core has set sufficiently, the continuous strip is cut intoshorter lengths or even individual boards or panels of prescribedlength. Once again, it is important for there to be good adhesionbetween the paper sheets and the set, but still wet, gypsum core or thecutting action will pull the edges of the paper facing sheet away fromthe gypsum core.

[0008] After the cutting step, the gypsum boards are separated andgrouped through a series of belts and rollers and then flipped overbefore being fed into drying ovens or kilns where the boards are driedso as to evaporate excess water. The hydration from hemihydrate togypsum must be essentially complete at this point, normally between 7and 15 minutes after mixing. When the gypsum boards are accelerated,flipped and fed into the drying ovens, the boards are subjected to avariety of stresses that can cause the facing to peel away from thegypsum core of the boards unless there is good adhesion between the set(but still wet) gypsum core and the facing material. Inside the dryingovens, the boards are blown with hot drying air at speeds up to 4000feet/minute which can cause further delamination of the paper facing ifthere is not good wet adhesion between the gypsum and the paper liners.When portions of the facing sheets delaminate from the gypsum coreduring drying in the oven, the liner becomes entangled in the rollersand the gypsum crumbles as it dries which jams the oven and requiresfrequent shut downs of the line while the loose gypsum is cleaned out ofthe ovens. The gypsum boards are dried in the ovens for anywhere from 30to 75 minutes. After the dried gypsum boards are removed from the ovens,the ends of the boards are trimmed off and the boards are cut to desiredsizes.

[0009] The fully dried gypsum adheres well to the paper facing sheetmaterials as long as the gypsum board is kept dry. However, paper facinghas a number of inherent properties that can be detrimental in a gypsumwallboard product. As discussed above, paper facing material (sometimescalled a paper liner) is made of cellulose which serves as a nutrientfor fungus growth. Paper facing also is not as strong or abrasionresistant as needed for certain construction applications. In addition,because the strength of paper differs significantly depending on thedirection in which the strength is measured, paper facing must berelatively thick in order to achieve satisfactory multidirectionalstrength. Paper faced gypsum-board products also suffer from a lack ofabrasion resistance. Paper facing used on conventional gypsum boardbecomes especially weak and subject to delamination from the gypsum corewhen the paper becomes damp due to leaks or high humidity.

[0010] Paper-faced gypsum boards must generally be coated with anothermaterial, such as paint or a wallcovering material, in order to achievesufficient abrasion resistance. For example, paper-faced wallboard isoften covered with vinyl wallcovering, a hard plastic sheet, or aplastic film when used in high traffic areas. Unfortunately, suchcoatings and coverings tend to trap moisture inside the wall where itcan precipitate fungus growth.

[0011] Canadian Patent No. 1,189,434 discloses gypsum panels made with afacing of a moisture vapor permeable spunbonded nonwoven material.Canadian Patent No. 1,189,434 discloses gypsum panels faced with Tyvek®spunbonded olefin sheet material. Tyvek® is a registered trademark ofE.I. du Pont de Nemours and Company of Wilmington, Del. Tyvek® sheetsare made by solution flash-spinning polyethylene to form fineplexifilamentary fibril structures that can be thermally bonded to formsheet material. U.S. Pat. No. 5,704,179 discloses gypsum board facedwith mats of fiberglass or synthetic resin fibers. While the panelsdisclosed in these patents eliminate naturally occurring organic matterfrom the facing sheets of the gypsum board, these patents are notdirected to reducing or eliminating fungus growth. Accordingly, thepatents do not disclose removal of nutrients from the gypsum core orother enhancements needed to reduce fungus growth in the gypsum board.

[0012] In addition, while it has been possible to produce gypsum boardsfaced with polymeric fibrous sheet materials on a small laboratoryscale, it has proven difficult to produce gypsum boards faced with suchsheets on a commercial scale. This is because the adhesive strengthbetween conventional fibrous synthetic fibrous sheets and the wet gypsumcore (known as wet adhesion) tends to be low. Thus, the facing peelsaway from the gypsum core during various points in the productionprocess before the boards are fully dried in the drying ovens.

[0013] There is a need for a process by which gypsum board that is freeof fungus nutrients such as cellulose, starch, and natural fibers thatcan be manufactured on a commercial basis. There is also a need for agypsum board that does not trap mold supporting moisture. Finally, thereis a need for gypsum boards that actually include substances thatprevent the growth of fungus.

BRIEF SUMMARY OF THE INVENTION

[0014] The present invention is directed to a fungus resistant gypsumboard, comprising: a first polymeric fibrous sheet, the first sheethaving a first surface and opposite first and second edges; a secondpolymeric fibrous sheet, the second sheet having a first surface andopposite first and second edges; a gypsum core sandwiched between thefirst and second nonwoven sheets, the gypsum core containing less than0.03% by weight, based on the weight of the dry gypsum core, offormulation additives that serve as fungus nutrients; and a syntheticadhesive on the first and second edges of said second sheets, thesynthetic adhesive adhering the first edge of said first nonwoven sheetto the first edge of the second nonwoven sheet, and adhering the secondedge of the first nonwoven sheet to the second edge of the secondnonwoven sheet. Preferably, the gypsum core contains less than 0.5% byweight, based on the weight of the dry gypsum core, of fungus nutrients.

[0015] According to a preferred embodiment of the invention, the gypsumcore contains a fungicide such as a metal/inorganic derivative. Morepreferably, the fungicide is boric acid, and the gypsum core iscomprised of between 0.04 and 0.25 weight percent, based on the weightof the dry gypsum core, of boric acid.

[0016] According to a preferred embodiment of the invention, the firstand second polymeric fibrous sheets are nonwoven sheet. In the preferredembodiment of the invention, the first surface of the first nonwovensheet and the first surface of the second nonwoven sheet adhere to saidwet gypsum core with an adhesive strength of at least 7.5 lb. The firstsurface of the first nonwoven sheet and the first surface of the secondnonwoven sheet have pores containing set gypsum of the gypsum coreintertwined with the fibers in the first and second nonwoven sheets. Thesheets may be comprised of a needle punched staple fiber sheet, ahydroentangled fibrous sheet, or a spunbond sheet. Alternatively, thefirst surface of the first and second nonwoven sheets may be coated witha primer layer of a high density gypsum slurry having a density that isat least 1.1 times the density of the gypsum core. Preferably, the firstand second sheets adhere to said wet gypsum core with an adhesivestrength of at least 10 lb.

[0017] The present invention is also directed to a process formanufacturing a gypsum-based substrate. The process includes the stepsof: adding calcined gypsum, formulation additives and water to a mixer,the mixture containing less than 0.02% by weight, based on the weight ofthe total slurry mix, of formulation additives that serve as fungusnutrients; mixing the gypsum and water in the mixer to produce a gypsumslurry that is comprised of 50 to 65 weight percent gypsum; providing afirst polymeric fibrous sheet, the first sheet having a first surfaceand opposite first and second edges; pouring the gypsum slurry from themixer onto the first surface of the first sheet and spreading the gypsumslurry over the first surface of the first sheet; providing a secondpolymeric fibrous sheet, the second sheet having a first surface andopposite first and second edges; applying a synthetic adhesive on thefirst and second edges of the second sheet; placing the first surface ofthe second sheet over the gypsum slurry that has been spread over thefirst surface of the first sheet; adhering the adhesive on the firstedge of the second sheet to the first edge of the first sheet, andadhering the adhesive on the second edge of the second sheet to thesecond edge of the first sheet; enclosing the gypsum slurry between thefirst and second sheets to bring the slurry into intimate contact withthe first and second sheets and form an elongated strip of gypsum slurrysandwiched between the first and second sheets; allowing the elongatedstrip of gypsum slurry to set up and harden to form a stiff elongatedstrip having a solid, wet gypsum core sandwiched between the first andsecond sheets; cutting the stiff elongated strip into gypsum board ofdesired length; drying the gypsum board in a dryer to remove excesswater from the gypsum boards.

[0018] In the process of the invention, the gypsum slurry contains lessthan 0.33% by weight, based on the weight of the total slurry, of fungusnutrients. After the elongated strip of gypsum slurry has set up andhardened to form a stiff elongated strip having a solid, wet gypsum coresandwiched between the first and second sheets, the first and secondsheets preferably adhere to the wet gypsum core with and adhesivestrength of at least 7.5 lb. According to the more preferred process ofthe invention, the first and second sheets are nonwoven sheets. It ispreferred that the first surface of the first nonwoven sheet and thefirst surface of the second nonwoven sheet have open pores betweenfibers of sufficient size for the gypsum slurry to enter the pores andbecome intertwined with the fibers in the sheets when the gypsum slurryis enclosed between the first and second nonwoven sheets. It isdesirable that the first and second sheets each have a mean flow poresize, measured according to ASTM F316-86, of at least 8.0 microns, andmore preferably in the range of 8.7 to 40 microns.

BRIEF DESCRIPTION OF THE DRAWING(S)

[0019] The accompanying drawings, which are incorporated in andconstitute a part of this specification, illustrate the presentlypreferred embodiments of the invention and, together with thedescription, serve to explain the principles of the invention.

[0020]FIG. 1 is a schematic representation of a portion of the processof the invention.

[0021]FIG. 2 is a cross-sectional view of a gypsum-based substrate madeaccording to the invention.

[0022]FIG. 3 is a schematic representation of another portion of theprocess shown in FIG. 1.

DEFINITIONS

[0023] As used herein, “fungus nutrients” means carbohydrate orcellulosic based materials, or other organic materials which arebiodegradable by fungi commonly found in building constructionmaterials.

[0024] As used herein, “fungicide” means a group of materials thatdestroy fungi or inhibits the growth of fungi. Fungicides includesynthetic compounds that are sulfur containing, halogens containing,metal containing, aliphatic, aromatic (phenol compounds andderiviatives), naphthol, quinoline, and imidazole derivatives.

[0025] As used herein, “formulation additives that serve as fungusnutrients” means raw materials that are used to manufacture gypsum boardand become incorporated into a final gypsum board product whichmaterials fall within the definition of “fungus nutrients.”

Test Methods

[0026] In the description above and in the non-limiting examples thatfollow, the following test methods were employed to determine variousreported characteristics and properties. ASTM refers to the AmericanSociety for Testing and Materials.

[0027] Mold Resistance of wallboard samples was measured according toASTM D3273 using 3.75 inch by 4 inch (9.5 cm by 10.2 cm) wallboardsamples, prepared as described in Example 1 below. The wallboard sampleswere tested in triplicate by suspending the samples above a mold-soilinoculum in an environmental humidity chamber equipped with a fan tocirculate the mold spores, constructed as detailed in ASTM D3273. Awhite pine control was also tested, as described in ASTM D3273. The soilinoculum for the study was prepared by seeding the incubating soil withthree strains of fungi: Aureobasidium pullulans (ATCC 9348), Aspergillusniger (ATCC 6275), Penicillium Sp. (ATCC 9849). ATCC refers to theAmerican Type Culture Collection, 12301 Parklawn Drive, Rockville, Md.20852The inside of the chamber was maintained at between about 90% to100% relative humidity and a temperature of 88° F.-92° F. (31° C.-33°C.). The test samples were continuously equilibrated and challenged withmold for 4.5 weeks, during which time the samples were rated each weekon a scale of 0 to 10 using photographic standards (ASTM D3274). Arating of 10 indicates undetectable mold growth and a rating of 0indicates substantial mold growth. Any sample ratings that differed bymore than an increment of 2 from the others for a particular examplewere disregarded and the average of the ratings was calculated based onthe remaining data points.

[0028] At the conclusion of the study, the gypsum board samples wereremoved and microscopically examined at a magnification of 50× todistinguish mold growth from any soil particles that may havecontaminated the board during the test. To further differentiate moldfrom soil particles, chlorine bleach was applied to the spots inquestion. Black spots disappearing on contact with the bleach wereconsidered to be mold whereas spots which were unaffected by the bleachsolution were considered to be soil particles. After microscopicexamination and treatment with bleach, the boards were rated again on ascale of 0 to 10 using photographic standards (ASTM D3274). Any sampleratings that differed by more than an increment of 2 from the others fora particular example were disregarded and the average of the ratings wascalculated based on the remaining data points and reported in Table 1 asthe final rating.

[0029] Wet Adhesion was measured using an Instron tensile testeraccording to the following procedure.

[0030] Gypsum boards were prepared using a mold comprising a laminatedboard having three aluminum rails of ½ inch height (12.7 mm) screwedthereto to define three sides of a rectangular mold with one open end.The aluminum rails were sized to form a mold having a length of about 20inches (50.8 cm) and a width of 3.75 inches (9.53 cm). With one of thelonger side rails removed, a nonwoven sheet having a length of 19 inches(48.3 cm) and a width of 5.5 inches (14.0 cm) was placed on the bottomof the mold to act as a liner on the first side of the gypsum board.After re-screwing the 20 inch (50.8 cm) rail to the bottom of the mold,1.75 inches (4.44 cm) of the 5.5 inch (14.0 cm) liner width extendedoutside of the mold, underneath the aluminum rail. The portion of theliner extending outside the mold forms an overhanging portion of theliner on the final gypsum board, which is inserted into the clamp of theInstron testing machine during wet adhesion testing. Immediately aftermixing, as described in the examples below, a gypsum slurry was pouredinto the mold onto the nonwoven liner and spread evenly over the surfacethereof. A second piece of nonwoven sheet material having dimensions ofabout 20 inches (50.8 cm) by 3.75 inches (9.53 cm) was placed on top ofthe gypsum slurry to act as a liner on the second side of the gypsumboard. The board was allowed to sit at room temperature for 20 minutesto allow the gypsum to set. The temperature of representative mixes weremonitored to ensure that the hydration was complete within this timeframe (the temperature of the mix rises during hydration, then holdssteady, and finally drops once hydration was complete). Hydration timesof 16 to 18 minutes were recorded for the boards produced in theexamples below. The boards were removed by unscrewing and removing theside rail which was on top of the overhanging section of liner andsliding each board out of the mold. The boards were flipped over so thatthe first side having the overhanging liner was on the top surface.

[0031] Immediately after the gypsum had set, each board was cut, using autility knife, into three, four or five 3 inch (7.6 cm) by 3.75 inch(9.53 cm) sections (with the top liner having dimensions of 3 inches(7.6 cm) by 5.5 inches (14 cm) due to the overhang). The top nonwovenliner on each board section was cut parallel to the 3.75 inch (9.53 cm)side into three 1 inch (2.54 cm) wide strips and each strip was cut inthe perpendicular direction such that the length of the liner section tobe pulled off the board was 2 inches (5.1 cm) (in addition to the 1.75inch (4.45 cm) overhang). The time taken to cut the board and the linerstrips was no more than about 10 minutes.

[0032] The board was clamped in the Instron machine and the center stripwas pulled from each board section with the Instron set at 20 or 50pounds force (89 or 222 Newtons). During testing, the liner strip waspulled from the board in the direction parallel to the length of theliner. The wet adhesion was measured as the force in pounds at which the2 inch length of liner was completely pulled away from the board.Testing of all of the three to five samples was completed within 5minutes of testing the first sample. The wet adhesion is reported as theaverage (+/− standard deviation) of the three to five samples tested foreach board.

[0033] Basis weight was determined by ASTM D-3776, which is herebyincorporated by reference, and is reported in g/m².

[0034] Maximum, minimum, and mean flow pore sizes were measured for thenonwoven liners on a Coulter Porometer II according to ASTM F316-86using Porofil wetting fluid, available from Coulter. The max pore sizeis an indicator of the diameter of the largest pore channels in thedistribution of pore sizes supporting flow through the web. The meanflow pore size is an indicator of the mean pore channel diameter for thepores supporting the total flow. The minimum pore size is an indicatorof the minimum pore channel diameter for the pores supporting the totalflow through the web. Pore size calculations were made using a sizefactor of 0.64, a tortuosity factor of 1.00, and a sample thickness of10 microns.

DETAILED DESCRIPTION OF THE INVENTION

[0035] Reference will now be made in detail to the presently preferredembodiments of the invention, examples of which are illustrated below.The present invention is directed to a gypsum board product that isresistant to the growth of fungus. The invention is also directed to animproved process for manufacturing gypsum board having a gypsum corelined with a synthetic nonwoven facing material without the addition oruse of materials or additives that may serve as fungus nutrients.

[0036] As discussed in the background section above, gypsum wallboard istraditionally manufactured by a continuous process. A commercial processfor manufacturing the fungus resistant gypsum board material of theinvention is shown in FIG. 1. As shown in FIG. 1, a gypsum slurry isfirst generated in a mechanical mixer 10 by mixing calcium sulphatehemihydrate (also known as calcined gypsum), water, water reducingagents, foam, bonding agents, and set control agents. The foam is amixture of water, a foaming agent such as alkyl sulfate/alkyl ethersulfate mixtures, and air. Other additives, such as anti-burning agents,can be added to the slurry as needed. Formulation additives that serveas fungus nutrients, such as cellulose fibers or starch, preferablycomprise less than 0.02% of the gypsum slurry, which corresponds to lessthan 0.03% by weight of the dried gypsum core. More preferably, thereare no formulation additives that are fungus nutrients. In addition, thegypsum used in the slurry is preferably comprised of less than 0.5%, byweight of the gypsum, of naturally occurring organic contaminants (suchas plant or animal matter) that may serve as a fungus nutrient.According to a preferred embodiment of the invention, the gypsum slurryincludes a fungicide such as a metal/inorganic derivative. Morepreferably, the gypsum slurry includes between 0.025% and 0.17% byweight boric acid, based on the total weight of the gypsum slurry.

[0037] The gypsum slurry 12 is deposited on the central portion of acontinuously advancing first polymeric fibrous sheet 14. The edges ofthe first sheet 14 are folded upward. As can be seen in thecross-sectional view of FIG. 2, each of the ends of the upturned edgesof the sheet 14 are folded toward each other along folds a shortdistance, depending on the thickness of the board, from each of thefirst folds so as to form strips 16 and 18 that are substantiallyparallel to the bottom of the sheet. An upper continuously advancingsecond polymeric fibrous sheet 20, with a an adhesive applied onopposite edges of its bottom surface as adhesive strips 22 and 23, islaid over the gypsum slurry such that the edge paste strips 22 and 23contact the folded over strips 16 and 18 of the first sheet 14.According to the invention the adhesive is material, such as a syntheticpressure sensitive polymeric adhesive that does not act as a fungusnutrient. Preferably, the adhesive strips 22 and 23 are comprised offast tacking polyvinyl alcohol based adhesives, synthetic resin basedadhesives, or hot melt adhesives. The polymeric fibrous sheets 14 and 20may be woven or nonwoven synthetic sheets. Nonwoven sheets made of fiberforming thermoplastic polymers are preferred.

[0038] As can be seen in FIG. 1, the first and second nonwoven sheetsand gypsum slurry are passed between parallel upper and lower formingplates 26 or rolls in order to generate an integrated and continuousflat strip 30 of unset gypsum sandwiched between synthetic fibroussheets which are referred to as synthetic facing or liners. The strip 30is conveyed over a series of continuous moving belts 32 and rollers (notshown) for a period of 2 to 5 minutes during which time the gypsum core28 sets up. It is important that a good bond be formed quickly betweenthe wet gypsum and the fibrous sheets 14 and 20 because the strip 30 canmove at speeds in excess of 500 ft/min over a distances of 1200-2000feet, during which time the strip 30 is transferred between multiplebelts and rollers. During each transfer between belts and/or rolls, thestrip 30 is stressed in a way that can cause the synthetic facing todelaminate from the gypsum core 28 if the adhesion between the gypsumcore and the facing is not sufficient. Once the gypsum has set, thecontinuous strip 30 is cut into shorter lengths or individual boards orpanels 34 of prescribed length by means of the rotating serrated blades38 and 39. Once again, it is important for there to be good adhesionbetween the synthetic fibrous sheets 14 and 16 and the set, but stillwet, gypsum core 28. Otherwise the blades 38 and 39 pull the edges ofthe synthetic facing sheet 14 and 16 away from the gypsum core 28 as theblades rotate during the cutting process.

[0039] After the cutting step, the gypsum boards 34 are accelerated onrollers 36 to separate the boards from each other. The separated gypsumboards are then lifted from the line and flipped over by my means of aplurality of lifting arms. The boards are fed, with their top sidesdown, into drying ovens or kilns where the boards are dried so as toevaporate excess water. When the gypsum boards are accelerated, flippedand fed into the drying ovens, the boards are subjected to a variety ofstresses that would cause the synthetic facing to peel away from thegypsum core of the boards but for the excellent wet adhesion between theset (but still wet) gypsum core and the facing material that is obtainedby means of the process of the invention. Inside the drying ovens, theboards are blown with hot drying air at speeds up to 4000 feet/minute.The absence of loose edges where there is not good adhesion between thenonwoven synthetic sheets 14 and 16 and the wet gypsum core 28 meansthat the facing is not pulled away from the gypsum core by the hotdrying air. The gypsum boards are dried in the ovens for anywhere from30 to 75 minutes. After the dried gypsum boards are removed from theovens, the ends of the boards are trimmed off and the boards are cut todesired sizes.

[0040] According to the invention, steps can be taken to improve the wetadhesion between the setting gypsum slurry and the synthetic fibroussheet, that do not include the addition of fungus nutrients, such asstarch, to the gypsum panels. According to one preferred embodiment ofthe invention, wetting agents that do not act as fungus nutrients areadded to the gypsum slurry or applied to the synthetic nonwoven sheetbefore the gypsum slurry and nonwoven liner are brought into contactwith each other. Such agents include synthetic chemicals withhydrophilic and hydrophobic groups which are known to reduce surfacetension of aqueous solutions and reduce contact angles with hydrophobicsolids. A wide range of wetting agents will perform this function suchas soaps and detergents, or even the foaming agents which are describedabove for adding foam to the gypsum core.

[0041] A preferred wetting agent is polyvinyl alcohol (PVA). Whileeffective as a wetting agent, it can also be used to replace the starchthat is normally used in a conventional gypsum board manufacturingprocess to improve the bonding of the liner to the dried gypsum core.Polyvinyl alcohol is commonly used as an adhesive and it has now beenfound that during the drying process, the polyvinyl alcohol will migrateto the interface between the liner and gypsum core and improve thebonding of the liner to the dried gypsum core to the extent that starchis not needed. Indeed, PVA has been found to be a more effective drybond adhesive than the starch for synthetic fibrous polymeric liners.The starch, a fungus nutrient, can be replaced by PVA, a formulationadditive that does not serve as a fungus nutrient.

[0042] According to another preferred embodiment of the invention, thefirst and second synthetic sheets each have a first surfacecharacterized by pores or spaces formed between fibers, which pores areof sufficient size for a gypsum slurry to enter the pores and becomeintertwined with the fibers in the sheets so as to form a strongmechanical bond between the gypsum core and the fibrous synthetic sheetswhen the gypsum sets up. The gypsum slurry is deposited on this firstporous surface of the first sheet and the first porous surface of thesecond sheet is juxtaposed against the gypsum such that when the gypsumslurry is enclosed between said first and second nonwoven sheets, theslurry impregnates into the pores or spaces between the fibers on thesurfaces of the first and second fibrous sheets. According to thisembodiment of the invention, a strong bond is formed between the wetgypsum core and the sheets in the absence of naturally occurringadditives, such as starch, that can serve as fungus nutrients.Preferably, the first and second nonwoven sheets each have a mean flowpore size, measured according to ASTM F316-86, of at least 8.0 microns.According to a more preferred embodiment of the invention, the first andsecond nonwoven sheets each have a mean flow pore size, measuredaccording to ASTM F316-86, in the range of 8.7 to 40 microns. This rangeof pore sizes allows the wet, set gypsum layer to intertwine with thefibers of the synthetic fibrous liner, providing good wet adhesion,without the gypsum slurry penetrating completely through the nonwovenliner.

[0043] According to the invention, the first and second sheets may benonwoven sheets comprised of meltspun substantially continuous fibers,carded staple fiber webs, needle punched staple fiber webs,hydroentangled fibrous webs, or other porous nonwoven syntheticstructures. The fibers in the first and second nonwoven sheets arecomprised of synthetic melt spinnable polymer. The preferred fibers arecomprised of one or more of any of a variety of polymers or copolymersincluding polyethylene, polypropylene, polyester, aramids, nylon,elastomer, and other melt spinnable polymers. For example, the fibers ofthe first and second nonwoven sheets may be comprised of at least 50% byweight polyester polymer, such as poly(ethylene terephthalate),poly(propylene terephthalate), or poly(butylene terephthalate) polymer.Alternatively, the fibers may be comprised of at least 50% by weight ofa nylon polymer, a polyolefin polymer such as polyethylene orpolypropylene, or an elastomeric polymer such as polyurethane orco-polyether ester.

[0044] According to one especially preferred embodiment of theinvention, the first and second nonwoven sheets may be comprised ofsmall denier polymeric fibers that, when made into a sheet structure,form numerous very small pores. The fibers of such sheet can be meltspun and air drawn according to the process disclosed in U.S. Pat. No.5,688,468. Such nonwoven sheet may be a unitary fibrous sheet comprisedof melt spun substantially continuous filament polymer fibers whereinthe sheet has a basis weight of from 13 g/m² to 125 g/m² andsubstantially all of the fibers are melt spun fibers. The fibers in suchnonwoven sheets have a cross sectional area of between about 20 andabout 90 μm², and more preferably, of from about 25 to about 70 μm², andmost preferably from about 33 to about 60 μm². Such melt spunmicrofibers sheets have a tensile strength (in both the machine andcross directions), normalized for basis weight, of from 0.7 to 5N/(g/m²), and more preferably from 0.8 to 4 N/(g/m²), and mostpreferably from 0.9 to 3 N/(g/m²).

[0045] In another preferred embodiment of the invention, the surface ofthe synthetic nonwoven liner which contacts the gypsum slurry has atextured surface comprising depressions and/or protrusions. Suchtextured surfaces can be found in embossed woven and nonwoven fabrics(e.g. thermally point-bonded nonwoven fabrics) or embossed wovenfabrics. The gypsum slurry flows into the depressions or around theprotrusions on the textured surface and mechanically locks the gypsumlayer to the liner as the gypsum layer expands during setting.Preferably the depressions or protrusions have dimensions in the rangeof about 50 to 2000 microns in the plane of the liner and from about 30to 500 microns in depth with between 20 and 100 depressions/protrusionsper square centimeter. More preferably, the dimension of the protrusionsin the plane of the liner is between about 100 and 1000 microns, thedepth of the protrusions is between about 200-500 microns, and there arebetween 30 and 75 depressions/protrusions per square centimeter. Thedimensions of the protrusions/depressions can be measured by microscopicanalysis using scanning electron microscopy techniques known in the art.

[0046] According to another preferred embodiment of the invention, theprocess of the invention may include the steps of coating the firstsurface of each of the first and second synthetic sheets with a thincoating of a dense gypsum slurry. Preferably, the first surface of saidfirst sheet and said first surface of said second sheet are coated witha layer of a high density gypsum slurry having a density that is 1.1 to3 times the density of the gypsum slurry used to form the core of thegypsum board. Preferably, the dense gypsum layer has a thickness in therange of {fraction (1/32)} to ⅛ inch and has a dry density of betweenabout 0.70 and 1.72 g/cc (corresponding to a wet density of betweenabout 1.06 to about 1.98 g/cc). The gypsum slurry density may becalculated based on a density of water of 1 g/cc and a gypsum density of2.32 g/cc or can be measured using methods known in the art. Typicalcommercial gypsum board core densities are approximately 0.96 g/cc (wet)and 0.63 g/cc (dry). Gypsum boards lined with a synthetic polymericfibrous liner having a high density layer adjacent the liner can beproduced using methods known in the art for paper liners. For example,the high density gypsum slurry may be coated onto a synthetic nonwovenliner using the roller-coating apparatus and method described in U.S.Pat. No. 5,879,486, which is hereby incorporated by reference.Alternately, a defoaming agent can be applied to the surface of thesynthetic polymeric nonwoven liner which results in an increase in thegypsum density immediately adjacent the liner, as described in U.S. Pat.No. 4,327,146, which is hereby incorporated by reference. In paper-facedgypsum boards, the paper has good wet adhesion to the gypsum slurry andthe high density gypsum layer is used to improve the dry bond betweenthe paper liner and the gypsum. In the current invention, the use of ahigh density gypsum layer results in improved wet adhesion between thesynthetic fibrous liner and the gypsum slurry, allowing the board to bemanufactured using conventional gypsum board manufacturing processes.

EXAMPLES

[0047] The following non-limiting examples are intended to illustratethe product and process of the invention and not to limit the inventionin any manner.

Example 1

[0048] A gypsum slurry was prepared by pre-blending, in a plastic bag,400 grams of General Purpose White Molding Plaster (available from USGCorporation), a beta-type hemihydrate plaster similar to that used in acommercial gypsum board factory, and 0.67 grams of a very finely groundgypsum accelerator having an average particle size of less than 2microns. The pre-blended powder was then sifted over a period ofapproximately 2 minutes onto the surface of a polyvinyl alcohol (PVA)solution contained in a Waring blender, allowing the gypsum to wet outand fall to the bottom of the blender. The PVA solution comprised 305 mlof a stock solution prepared by dissolving 22.1 grams of Elvanol® 90-50polyvinyl alcohol (available from E.I. du Pont de Nemours and Company)in 1000 ml of water, heating to dissolve the polyvinyl alcohol, andcooling the resulting solution at room temperature for at least 24hours. Immediately after the addition of the pre-blended powder to thePVA solution was completed, foam that had been prepared by blending 65ml of a 0.5 weight percent solids solution of an alkyl sulphate/alkylether sulphate blend (Cedepal FA-406, manufactured by Stepan Chemicals)in a separate Waring blender for approximately 2 minutes was poured ontop of the water/solids mixture and the blender turned on for 10seconds. The resulting gypsum slurry was used immediately for preparingthe gypsum boards.

[0049] Half-inch thick gypsum boards were prepared by coating the gypsumslurry on a 3.75 inch by 16 inch (9.5 cm by 40.6 cm) sheet of Tyveke®1058D flash-spun high density polyethylene (available from E.I. du Pontde Nemours and Company) placed on the bottom of a mold and covering thegypsum slurry with a second Tyvek® 1058D sheet. Rubber gloves were wornthroughout the board preparation process to avoid contamination of theliner surface with oils and dirt. After casting the board, it wasremoved from the mold, and dried in a General Signal Blue M Seriesforced air oven at 127° C. for 90 minutes, after which the oventemperature was ramped down to 75° C. over a period of 45 minutes. Theoven was then turned off and allowed to cool to room temperatureovernight. The dried gypsum board was removed from the oven and cut intofour equal 4 inch×3.75 inch sections for mold testing. A {fraction(1/16)} inch hole was drilled about ⅛ inch from one of the 3.75 inchedges, centered on the edge, used for suspending the boards in theenvironmental chamber during mold testing.

[0050] The boards were tested for resistance to mold growth as describedabove and the results are reported in Table 1 below. There was noevidence of delamination of the nonwoven liner during testing.

[0051] Boards were prepared for wet adhesion measurements and tested asdescribed above. An average wet adhesion of 7.1±1.3 pounds-force(31.5±5.8 Newtons) was obtained.

Example 2

[0052] Gypsum boards were prepared as described in Example 1, exceptthat 16.7 ml of Aqualite 70 wax emulsion (available from Monsey Bakor,Inc.) was added to the Waring Blender containing the 305 ml of PVAsolution prior to adding the pre-blended gypsum powder.

[0053] The boards were tested for resistance to mold growth as describedabove and the results are reported in Table 1 below. There was noevidence of delamination of the nonwoven liner during testing.

Example 3

[0054] Gypsum boards were prepared as described in Example 1, exceptthat the lining was a thermally point bonded spunbonded polypropylenefabric having a basis weight of 2.1 oz/yd2 containing 2 weight %pigment. The fibers had an effective diameter of about 10 microns. Thegypsum board was prepared with the embossed sides of the liner facingthe gypsum slurry.

[0055] The boards were tested for resistance to mold growth as describedabove and the results are reported in Table 1 below. There was noevidence of delamination of the nonwoven liner during testing.

Example 4

[0056] Gypsum boards were prepared as described in Example 1, exceptthat the lining was a point bonded spunbonded polyester fabric having abasis weight of 1.9 oz/yd², and comprising fibers having an effectivediameter of approximately 8.6 microns. The spunbonded liner wasthermally point bonded between an engraved oil-heated metal calenderroll and a smooth oil heated metal calender roll. The engraved roll hada chrome coated non-hardened steel surface with a diamond pattern havinga point size of 0.466 mm², a point depth of 0.86 mm, a point spacing of1.2 mm, and a bond area of 14.6%. The point bonded sheet had a minimumpore size of 14.69 microns, maximum pore size of 70.63 microns, and amean flow pore size of 29.01 microns. The gypsum board was prepared withthe embossed sides of the liner facing the gypsum slurry.

[0057] The boards were tested for resistance to mold growth as describedabove and the results are reported in Table 1 below. There was noevidence of delamination of the nonwoven liner during testing.

[0058] Boards were prepared for wet adhesion measurements and tested asdescribed above. An average wet adhesion of 12.3±2.2 pounds-force (55±10Newtons) was obtained.

Example 5

[0059] Gypsum boards were prepared as described in Example 1, exceptthat the lining was Sontara® E88-320 spunlaced polyester having a basisweight of 4 oz/yd² (available from E.I. du Pont de Nemours and Company).

[0060] The boards were tested for resistance to mold growth as describedabove and the results are reported in Table 1 below. There was noevidence of delamination of the nonwoven liner during testing.

[0061] Boards were prepared for wet adhesion measurements and tested asdescribed above. An average wet adhesion of 10.6±6.6 pounds-force(47.2±29 Newtons) was obtained.

Comparative Example A

[0062] In this example, a commercial paper-lined Sheetrock® gypsumboard, manufactured by USG Corporation was tested for mold resistance.

[0063] The boards were tested for resistance to mold growth as describedabove and the results are reported in Table 1 below. The paper liner wasobserved to start to delaminate from the gypsum core during testing.TABLE 1 Fungus Resistance Ratings for Gypsum Boards Micro- Core ASTMASTM scopic Final Example Liner Additives Ratings Average Ratings Rating1 Tyvek ® PVA 8, 8, 10 9 10, 10, 10 1058D 10 2 Tyvek ® PVA/Wax 10, 8, 68 10, 10, 10 1058D emulsion 6 3 Spunbond PVA 8, 10, 8 8 6, 10, 10 PP 104 Spunbond PVA 8, 8, 8 8 10, 10, 10 PE 10 5 Sontara ® PVA 10, 8, 8 9 8,10, 10 E88-320 10 Comp. A Paper Commer- 2, 10, 4 3 2, 10, 4 3 cial BoardWhite — — 4 4 4 4 Pine Control

[0064] It can readily be seen that the gypsum boards made in accordancewith the invention without the introduction of materials that serve asfungus nutrients are far more resistant to mold and fungus growth thanis the conventional gypsum board of Comparative Example A. It will beapparent to those skilled in the art that modifications and variationscan be made in process and gypsum board material of invention. It isintended that all matter contained in the foregoing description,drawings and examples shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. A fungus resistant gypsum board, comprising: afirst polymeric fibrous sheet, said first sheet having a first surfaceand opposite first and second edges; a second polymeric fibrous sheet,said second sheet having a first surface and opposite first and secondedges; a gypsum core sandwiched between said first and second nonwovensheets, said gypsum core containing less than 0.03% by weight, based onthe weight of the dry gypsum core, of formulation additives that serveas fungus nutrients; and a synthetic adhesive on the first and secondedges of said second sheets, said synthetic adhesive adhering the firstedge of said first nonwoven sheet to said first edge of said secondnonwoven sheet, and adhering the second edge of said first nonwovensheet to the second edge of said second nonwoven sheet.
 2. The board ofclaim 1 wherein said gypsum core contains less than 0.5% by weight,based on the weight of the dry gypsum core, of fungus nutrients.
 3. Theboard of claim 1 wherein said gypsum core contains a fungicide.
 4. Theboard of claim 3 wherein said fungicide is a metal/inorganic derivative.5. The board of claim 4 wherein said fungicide is boric acid, and thegypsum core is comprised of between 0.04 and 0.25 weight percent, basedon the weight of the dry gypsum core, of boric acid.
 6. A fungusresistant gypsum-based substrate, comprising: a first polymeric fibrousnonwoven sheet, said first nonwoven sheet having a first surface andopposite first and second edges; a second polymeric fibrous nonwovensheet, said second nonwoven sheet having a first surface and oppositefirst and second edges; a wet and hydrated gypsum core sandwichedbetween said first and second nonwoven sheets, said gypsum corecontaining less than 0.02% by weight, based on the weight of the wetgypsum core, of formulation additives that serve as fungus nutrients,and less than 0.33% by weight, based on the weight of the wet gypsumcore, of fungus nutrients; a synthetic adhesive on the first and secondedges of said second nonwoven sheet, said synthetic adhesive adheringthe first edge of said first nonwoven sheet to said first edge of saidsecond nonwoven sheet, and adhering the second edge of said firstnonwoven sheet to the second edge of said second nonwoven sheet; andwherein said first surface of said first nonwoven sheet and said firstsurface of said second nonwoven sheet adhere to said wet gypsum corewith an adhesive strength of at least 7.5 lb.
 7. The gypsum board ofclaim 6 wherein said first surface of said first nonwoven sheet and saidfirst surface of said second nonwoven sheet have pores containing setgypsum of said gypsum core intertwined with the fibers in the first andsecond nonwoven sheets.
 8. The gypsum board of claim 7 wherein saidfirst surface of said first nonwoven is comprised of a web selected fromthe group of needle punched staple fiber sheets, hydroentangled fibroussheets, and spunbond sheets.
 9. The gypsum board of claim 6 wherein saidfirst surface of said first nonwoven sheet and said first surface ofsaid second nonwoven sheet are coated with a primer layer of a highdensity gypsum slurry having a density that is at least 1.1 times thedensity of the gypsum core.
 10. The gypsum board of claim 6 wherein saidfirst and second sheets adhere to said wet gypsum core with an adhesivestrength of at least 10 lb.
 11. A process for manufacturing agypsum-based substrate, comprising the steps of: adding calcined gypsum,formulation additives and water to a mixer, said mixture containing lessthan 0.02% by weight, based on the weight of the total slurry mix, offormulation additives that serve as fungus nutrients; mixing the gypsumand water in the mixer to produce a gypsum slurry that is comprised of50 to 65 weight percent gypsum; providing a first polymeric fibroussheet, said first sheet having a first surface and opposite first andsecond edges; pouring said gypsum slurry from said mixer onto the firstsurface of said first sheet and spreading the gypsum slurry over saidfirst surface of said first sheet; providing a second polymeric fibroussheet, said second sheet having a first surface and opposite first andsecond edges; applying a synthetic adhesive on the first and secondedges of said second sheet; placing said first surface of said secondsheet over the gypsum slurry that has been spread over the first surfaceof said first sheet; adhering the adhesive on the first edge of saidsecond sheet to said first edge of said first sheet, and adhering theadhesive on the second edge of said second sheet to said second edge ofsaid first sheet; enclosing the gypsum slurry between said first andsecond sheets to bring the slurry into intimate contact with said firstand second sheets and form an elongated strip of gypsum slurrysandwiched between said first and second sheets; allowing said elongatedstrip of gypsum slurry to set up and harden to form a stiff elongatedstrip having a solid, wet gypsum core sandwiched between said first andsecond sheets; cutting said stiff elongated strip into gypsum board ofdesired length; and drying said gypsum board in a dryer to remove excesswater from the gypsum boards.
 12. The pcocess of claim 11 wherein in thestep of adding calcined gypsum, formulation additives and water to amixer, said mixture contains less than 0.33% by weight, based on theweight of the total slurry, of fungus nutrients.
 13. The process ofclaim 11 wherein after the elongated strip of gypsum slurry has set upand hardened to form a stiff elongated strip having a solid, wet gypsumcore sandwiched between said first and second sheets, said first andsecond sheets adhere to said wet gypsum core with and adhesive strengthof at least 7.5 lb.
 14. The process of claim 11 wherein said first andsecond sheets are nonwoven sheets.
 15. The process of claim 14 whereinsaid first surface of said first nonwoven sheet and said first surfaceof said second nonwoven sheet have open pores between fibers ofsufficient size for the gypsum slurry to enter the pores and becomeintertwined with the fibers in the sheets when the gypsum slurry isenclosed between said first and second nonwoven sheets.
 16. The processof claim 15 wherein said first and second sheets each have a mean flowpore size, measured according to ASTM F316-86, of at least 8.0 microns.17. The process of claim 16 wherein said first and second sheet eachhave a mean flow pore size, measured according to ASTM F316-86, in therange of 8.7 to 40 microns.
 18. The process of claim 15 wherein saidfirst surface of said first nonwoven sheet and said first surface of andsecond nonwoven sheets are comprised of a web selected from the group ofneedle punched staple fiber sheets, hydroentangled fibrous sheets, andspunbond sheets.
 19. The process of claim 12 wherein said first surfaceof said first sheet and said first surface of said second sheet arecoated with a primer layer of a high density gypsum slurry having adensity that is at least 1.1 times the density of the gypsum slurry. 20.The process of claim 14 wherein the dried gypsum board is comprised ofless than 0.03% by weight, based on the weight of the gypsum core, offormulation additives that serve as fungus nutrients, and less than 0.5%by weight, based on the weight of the gypsum core, of fungus nutrients.